In many industrial processes, control of film thickness is of critical importance. For the purposes of this discussion, the term "film" shall be used to denote a film, web, or sheet.
For example, the manufacture of photographic film requires the generation of a uniform layer of emulsion on a backing. From the point of view of process control, it is advantageous to be able to measure the film thickness during the film generation process rather than measuring the film in a quality control laboratory after the film has been manufactured. If samples are measured off-line, correction of any machinery malfunction cannot be performed until after a considerable volume of defective film has been processed. This leads to waste.
Prior art techniques for measuring film thickness may be divided into contact and non-contact methods. In one contact technique, a micrometer that comes in physical contact with both sides of the film is employed. These techniques have the disadvantage of physically deforming the film during the measurement. In addition, these techniques are difficult to apply for the on-line measurement of fast moving film webs.
Non-contact techniques based on the attenuation of a beam of atomic particles or radiation such as beta particles or gamma rays are also known to the prior art. For example, the attenuation of a beam of electrons by the film is used to determine the film thickness in one prior art method of this type. This methodology has two disadvantages. First, the system must be calibrated for each type of film, since the attenuation depends on the chemical composition and density of the film. Second, the system typically relies on a radioactive source to generate the particle beam. It is generally desirable to limit the use of radioactive material for cost, safety, and psychological reasons.
Methods for measuring the thickness of films using an optical autocorrelator are known also known to prior art. For the purposes of this discussion, an optical autocorrelator is defined to be an interferometer having a variable differential time delay. One embodiment of an optical autocorrelator is described, for example, in chapter 5 of Statistical Optics, by Joseph W. Goodman (John Wiley & Sons, 1985, pp. 157-170). Those skilled in the art are aware of the principles of operation of an optical autocorrelator, but certain principles will be clarified here because of their relevance to this patent. In an autocorrelating interferometer wherein light is split into two different paths and then recombined and directed to a photodiode, the detected light intensity is measured as a function of a parameter. This parameter can be the differential optical path length .alpha.L of the interferometer or it can be the differential time delay .alpha.t of the interferometer. These parameters are related by .alpha.L=n c .alpha.t, where c is the speed of light in vacuum and n is the group index of the medium (usually air) of the differential optical path. The detected light intensity expressed as a function of the differential time delay is called the coherence function of the input light. Hence, a receiver which determines the time delay between light reflected from different surfaces of a film performs the same function as a receiver which determines the path delay between light reflected from different surfaces of a film. Determining the spacing between peaks in the coherence function of the reflected light is yet another way to describe the same function.
A Michelson interferometer is an example of such an autocorrelator. An example of an apparatus for measuring film thickness which utilizes a Michelson interferometer is taught in U.S. Pat. 3,319,515 to Flournoy. In this system, the film is illuminated with a collimated light beam at an angle with respect to the surface of the film. The front and back surfaces of the film generate reflected light signals. The distance between the two reflecting surfaces is then determined by examining the peaks in the autocorrelation spectrum generated in a Michelson interferometer that receives the reflected light as its input. Unfortunately, this method can determine only the product of the group index of refraction and the film thickness. If a variation is detected in this quantity, additional measurements must be made to determine if the film composition has changed or the thickness has changed. The group index is defined to be the ratio of the propagation velocity of a light pulse in the medium relative to the velocity of propagation of the pulse in a vacuum.
Broadly, it is the object of the present invention to provide an improved apparatus and method for measuring the thickness of a film, sheet or web film.
It is a further object of the present invention to provide a system that does not require contact between the film and the measuring device.
It is a still further object of the present invention to provide a system that can accommodate flutter in the film.
It is yet another object of the present invention to provide a system that can determine both the group index and the film thickness when transparent films are measured.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of various embodiments of the invention and the accompanying drawings.